1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly to a method for forming an anti-reflective film pattern beneath a photoresist film so as to improve a photo contrast upon forming a pattern having a critical dimension by use of a lithography process.
2. Description of the Prior Art
In formation of a pattern having a critical dimension using a lithography process, a light hv incident on a photoresist film via a mask is reflected by the surface of a layer disposed beneath the photoresist film. The reflected light is then reflected by the surface of the photoresist film. This reciprocation of the reflected light is repeated several times, so that energy is transferred to the photoresist film.
FIGS. 1A and 1B are sectional views respectively illustrating a conventional method for forming a photoresist film.
In accordance with this method, first, a lower layer 14 and a photoresist film 13 are sequentially formed over a semiconductor substrate 10, as shown in FIG. 1A. Thereafter, the photoresist film 13 is exposed to a light 11 using a mask 12 formed with chromium patterns on its quartz substrate. During the exposure, the light 11 is partially reflected by the surface of photoresist film 13 while partially transmitting the photoresist film 13. The light transmitting the photoresist film 13 is reflected by the surface of lower layer 14. The reflected light which is denoted by the reference numeral 16 is reflected by the surface of the photoresist film 13. As such a reflection of light is repeated. several times, and the photoresist film is exposed to the light even at portions intended not to be exposed. During the exposure, the light reaching the mask 12 is diffracted at the edge of each chromium pattern of mask 12 while transmitting the mask 12. This diffracted light is then incident on a non-exposure portion of the photoresist film 13. This incident light which is denoted by the reference numeral 17 is then reflected by the surface of lower layer 14.
Thereafter, a development is carried out to remove the light-exposed portion of the photoresist film 13 and thereby to form a photoresist film pattern 13A. FIG. 1B shows a condition the photoresist film pattern 13A has notched portions 20 respectively formed at side wall portions of the pattern.
Where the line space between adjacent chromium patterns of the mask 12 shown in FIG. 1 is approximate to the wavelength of the exposure light, a severe diffraction phenomenon occurs when the light passes through the mask 12. This severe diffraction results in a severely degraded profile of the photoresist film pattern.
An intensity distribution of light emerging from the mask can be found from a modulation transfer function. The intensity distribution of light, that is, the modulation M can be expressed by the following equation: ##EQU1## where, "Imax" and "Imin" represent the maximum intensity of light and the minimum intensity of light, respectively.
Generally, the modulation M is dependent on the line space between adjacent chromium patterns formed on the mask. For example, the modulation M is reduced as the line space is decreased. At a lower modulation M, the intensity of light is inefficiently distributed in the photoresist film. In this case, no photoresist film pattern may be formed. Even if a photoresist film pattern is formed, it has no vertical surface at its edge.
Also, a modulation M exhibiting a low contrast results in a small process margin, and the manufacture of semiconductor devices becomes difficult.